Effect of electric field and humic acid on the mobility of biochar particles in soil

Biochar is a widely used material for the remediation of contaminated soils. However, the recovery of biochar after its application in soil remains a significant challenge. To regulate the migration of particles and identify a possible strategy for the recovery of biochar from soil, this study investigated the transport behavior of biochar microparticles (MPs) and nanoparticles (NPs) in real soil under different potential gradients (0, 0.5 and 1.0·V cm-1) and humic acid (HA) concentrations (0, 5 and 10 mg·L-1). It was demonstrated that the electric field and the presence of HA could enhance the mass recovery of biochar particles. Further improvement in the mobility of biochar was observed upon increasing potential gradient and HA content, which resulted in an increase in electrostatic repulsion between biochar and soil particles. In the optimal conditions, the recovery of biochar MPs and NPs in the effluent could be significantly increased to 46.3±1.2% and 72±1.2% at 1.0 V·cm−1 in the presence of 5 mg·L-1 HA. The transport of biochar particles in soil was well explained by the convection-diffusion equation (CDE) model. The enhanced migration was attributed to the increase of oxygen-containing functional groups on biochar particles by the electric field, which reduced the zeta potential (ζ-potential) of biochar and thus increased the electrostatic repulsion between them and the soil. Furthermore, the application of direct current reduced the size of biochar particles, facilitating biochar migration by reducing the size-blocking effect. The results of this study provide a basis for regulating the migration of biochar in soil.